Tributaries join the main stem at a confluence

- routing.tributaries lets an outlet keep extra surface courses; each is cut where its centerline first meets the stem and joined with a short connector
- A tributary must keep half the minimum course length and arrive at or above the stem's water
- Surface shape rejections carry the failing rule as their detail

probe/src/main/java/art/arcane/iris/probe/HydrologyPlannerProbe.java

@@ -229,7 +229,7 @@ public final class HydrologyPlannerProbe {
                         12D,
                         0.5D,
                         0.1D
-                , 1D),
+                , 1D, 0),
                 new HydrologyPlannerSettings.Surface(
                         true,
                         surfaceSources,
@@ -440,7 +440,7 @@ public final class HydrologyPlannerProbe {
                         12D,
                         0.5D,
                         0.2D
-                , 1D),
+                , 1D, 0),
                 new HydrologyPlannerSettings.Surface(
                         true,
                         surfaceSources,